The present invention relates to a process for the preparation of 3-aminomethyl-3,5,5-trimethylcyclohexylamine, hereafter also called isophoronediamine or abbreviated as IPDA, from 3-cyano-3,5,5-trimethylcyclohexanone, hereafter also called isophoronenitrile or abbreviated as IPN, by an aminating hydrogenation reaction with hydrogen and ammonia in the presence of a fixed bed catalyst. The process according to the present invention affords the continuous preparation of isophoronediamine in high yield and high purity.
Isophoronediamine is used as a starting material for the preparation of isophorone diisocyanate, which is used as an isocyanate component for polyurethane systems, as an amine component for polyamides, and as a hardener for epoxy resins. Isophoronediamine is conventionally prepared from isophoronenitrile, the carbonyl group being converted to an amino group and the nitrile group to an aminomethyl group in the presence of ammonia, hydrogen and conventional hydrogenation catalysts. The starting material, isophoronenitrile, can be obtained in a conventional manner by the addition of hydrogen cyanide onto isophorone (see U.S. Pat. No. 5,091,554 which is incorporated by reference in its entirety; DE-OS 39 42 371).
According to the process described in U.S. Pat. No. 3,352,913 (which is incorporated by reference in its entirety) for the preparation of isophoronediamine from isophoronenitrile, the hydrogenation is carried out in the presence of ammonia and in the presence of cobalt-, nickel-, iron- or noble metal-containing catalysts known per se, at 50.degree. to 150.degree. C. and at a pressure of at least 5 MPa. By way of example, the hydrogenation takes place in the presence of methanol as a solvent. Apart from the desired isophoronediamine, by-products, such as 3-aminomethyl-3,5,5-trimethylcyclohexanol (isophoroneamino alcohol, abbreviated as IPAA) in particular, are formed in relatively large amounts. The disadvantages of this process are found to be the low yield and a considerable proportion of by-products.
In the quest to obtain a higher yield of IPDA and minimize the unavoidable formation of IPAA, DE-OS 30 11 656 teaches a two-step process; in the first step, IPN is converted with excess ammonia, without a catalyst, to 3-cyano-3,5,5-trimethyliminocyclohexane which is hydrogenated in the second step to IPDA. The disadvantage of this process is that, as well as the actual hydrogenation reactor, a special imine formation reactor is required.
EP-B 0 042 119 (U.S. Pat. No. 4,429,157) proposes a further improvement to the process of producing IPDA, wherein, prior to reaction with ammonia and hydrogen in the presence of hydrogenation catalysts at temperatures of 10.degree. to 120.degree. C. and pressures of 0.1 to 30 MPa, the isophoronenitrile is subjected to a preliminary reaction with ammonia in the presence of inorganic and organic ion exchangers in the ammonium form as imine formation catalysts. Whereas the volume ratio of isophoronenitrile to ammonia in the imine (isophoroneimine or Schiff's base) formation step is said to be 1:0.5 to 20, this ratio is increased to 1:10 to 20 in the hydrogenation step. The economy of the process is compromised by the use of two different catalysts and the large excess of ammonia, which necessitates a very high pressure and hence an expensive hydrogenation apparatus.
EP-B 0 042 119 also discloses a Comparative Example in which isophoronenitrile and liquid ammonia are pumped into the top of a hydrogenation reactor charged with commercially available cobalt catalyst. The reaction system is kept at 270 bar with H.sub.2 ; a certain gas stream is set up and a certain amount of off-gas is withdrawn. The reaction mixture leaving the bottom of the reactor is then worked up by distillation. Despite approximately quantitative conversion of the isophoronenitrile, this embodiment produces only 48% of isophoronediamine, together with many by-products. EP-B 0 042 119 does not suggest passing a mixture containing isophoronenitrile, ammonia and organic solvent, instead of a mixture of isophoronenitrile and ammonia, over a trickle bed reactor at an essentially low pressure and without the need for a preliminary imine formation reaction.
EP-A 0 449 089 (CA 2,039,328) discloses another process for the preparation of isophoronediamine. In two spatially separated reaction chambers, a solution of isophoronenitrile in tetrahydrofuran is initially reacted with excess ammonia on acidic metal catalysts to form 3-cyano-3,5,5-trimethylcyclohexylimine and the reaction mixture is hydrogenated at high pressure in a second reaction chamber with hydrogen in the presence of excess ammonia on catalysts containing cobalt, nickel, ruthenium and/or other noble metals, and optionally basic components. By way of example, the mixture leaving the first reaction step is passed through the hydrogenation reactor from bottom to top; the reactor is operated as a bubble reactor. EP-A 0 49 089 does not give any indication of also using the reactor as a trickle bed reactor and directly feeding said reactor with a mixture of isophoronenitrile, ammonia and an organic solvent.
EP-A 0 394 967 teaches a process for the amination of carbonylnitriles and iminonitriles and also includes the preparation of isophoronediamine from isophoronenitrile. The starting material is initially converted to the aminonitrile at moderate temperatures under conditions of reductive amination, i.e. in the presence of hydrogen, ammonia and a hydrogenation catalyst; the nitrile group is then converted to an aminomethyl group at elevated temperature in the presence of a hydrogenation catalyst which is effective in the hydrogenation of nitrile groups. Although this process can be carried out at low pressures, the fact that a strictly observed temperature program has to be operated during both the reaction steps is regarded as a considerable disadvantage since it results in an appreciable lowering of the space-time yield and hence the economy of the process. In addition, unless special promoters are also used, the product quality does not satisfy the requirements of low by-products because the content of 3-cyano-3,5,5-trimethylaminocyclohexane, which cannot be separated off by distillation, is usually too high.
According to JP-A-4-300852, a pressure reduction in the preparation of isophoronediamine by the aminating catalytic hydrogenation of isophoronenitrile is also made possible by carrying out the hydrogenation in the presence of a supported ruthenium catalyst. However, JP-A-4-300852 refers only to conditions for suspension hydrogenation and not to conditions using a fixed bed reactor.
SRI International Report No. 1D, "Isocyanates" by YUREN CHIN (July 1983), gave a survey of an industrial process for the preparation of isophoronediamine from isophoronenitrile. In the process described therein, a mixture of isophoronenitrile, methanol and ammonia is passed with hydrogen over a fixed bed reactor containing a supported catalyst of cobalt-on-kieselguhr. In the embodiment described, the catalyst bed is always flooded (bubble column). The operating pressure is given as 150 bar. Also, hydrogen is used in excess and, after the reaction, this excess must be separated from the reaction mixture, purified, compressed and then recycled. The economy of this process is reduced both by the high operating pressure and by the technical cost of recycling the hydrogen. As established in comparative Example 3b below, this embodiment results in the increased formation of by-products which cannot easily be separated off, and to a lowering of the yield.